The present invention relates to a machine for sharpening the blades of ice skates.
As is well known, the bottom edge of an ice skate blade is required to have a somewhat concave transverse cross-section so that a sharp ridge extends along each of the two side edges. This shape is maintained by grinding the blade periodically with a grinding wheel, the periphery of which has a corresponding convex shape in transverse cross-section. A typical machine for this purpose has been disclosed, for example, in U.S. Pat. No. 3,735,533 issued May 29, 1973 to M. Salberg (Canadian patent No. 920,819 dated Feb. 13, 1973).
The present invention has the general objective of providing improvements in this class of machine and, in particular, seeks an improvement in the uniformity of grinding along the length of the blade. The depth of metal to be removed from the blade will vary, e.g. from about 2 thousandths of an inch, in the case of a blade requiring only minor sharpening, up to about 10 thousandths of an inch in the case of a badly deteriorated blade. Whatever the depth of cut that is selected, it is important that essentially the same depth of cut be maintained along the full length of the blade. If a reasonable approximation to this ideal is not maintained, after repeated sharpening operations the basic profile of the blade will become significantly modified.
One of the principal difficulties with prior machines of this type has been a tendency for them to remove more metal from the ends of the blade, i.e. near the toe and the heel, than from the central portion of the blade, during each pass of the grinding wheel along the blade. A skate blade is initially constructed with a convex profile when seen in side view. In the longitudinally central portion of the blade this convexity is comparatively slight. At the ends, the curvatures are sharper, especially at the toe. This profile is designed to maximise the performance of the skate and it is therefore desirable to maintain such profile throughout the life of the blade. Removal of excessive metal from either or both of the toe and heel portions will distort this profile and shorten the life of the skate.
The tendency towards removal of more metal from the end portions than from the center, flows from the fact that, when a grinding wheel is urged against a skate blade in a direction perpendicular to the general longitudinal direction of the blade and the portion of the blade engaged by the wheel does not extend at right angles to such perpendicular direction due to the curvature of the blade, such portion contacts the wheel at a radius inclined to such perpendicular direction. Thus, if a uniform force is applied to bias the wheel in the perpendicular direction throughout a complete pass of the wheel along the blade, the blade resists this applied force by a somewhat larger force acting along the inclined radius, since the action and reaction forces between the blade and the wheel can only act along a radius normal to the tangential direction of the contact between the blade and the wheel.
A proposal to compensate for this effective increase of force between the blade and the wheel (and hence increased depth of cut) at the ends of the blade by varying the perpendicular biassing force in accordance with the grinding resistance (sensed by measuring the power input to the motor driving the wheel), is disclosed in U.S. Pat. No. 4,235,050 issued Nov. 25, 1980 to J. H. Hannaford et al. (Canadian patent No. 1,118,514 issued Feb. 16, 1982). This proposed solution has, however, not been found reliable or satisfactory in practice.